Finally figured out how to setup prerender + nginx/passenger

nginx.conf

server {
  listen 80;
  ...
  
          location / {
                passenger_enabled on;
                root /home/deploy/current/public;
                include prerender.conf;
        }
}

prerender.conf

set $needPrerender "";

if ($request_uri ~ '_escaped_fragment_') {
  set $needPrerender "YES";
}

if ($http_user_agent ~* (googlebot|google.com|bingbot|bing.com|yandexbot|yandex.com|yahooseeker|yahoo.com|slurp|feedfetcher|blekkobot|crawler) ) {
  set $needPrerender "YES";
}

if ($http_accept ~* 'html') {
  set $needPrerender "${needPrerender}ES";
}

if ($needPrerender = "YES") {
  rewrite ^ /$scheme://$http_host$request_uri break;
  proxy_pass http://localhost:3000;
}